[BEAM-12815] Try to fix flaky Flink Post Commit

[damccorm]

Right now, Go flink Post commits are failing with java.io.IOException: Insufficient number of network buffers: required 17, but only 16 available. The total number of network buffers is currently set to 2048 of 32768 bytes each. You can increase this number by setting the configuration keys 'taskmanager.memory.network.fraction', 'taskmanager.memory.network.min', and 'taskmanager.memory.network.max'.

This is the same underlying issue I fixed in #17067 but it shows up again here because we have a different mechanism for spinning up flink for these post commit tests. This PR does 2 things:

1. It moves the memory buffer config used by the Go tests into our shared flink test config. This is verified working in the precommit checks.
2. It sets PflinkConfDir to point to this config directory in our post commit infra. This corresponds to a property in our flink gradle job server file where we start up flink -

   beam/runners/flink/job-server/flink_job_server.gradle

   Line 116 in 5a62228

   if (project.hasProperty('flinkConfDir'))

NOTE: The post commit changes (changing PflinkConfDir) aren't able to be tested because I can't actually repro the problem locally, and Jenkin's infra seems to run post commits against master (even when triggered against PRs). So this may not work, but the blast radius (potential to briefly break post-commits) is relatively small, and I'll quickly revert it if this makes things any worse. I do feel pretty confident it won't make things worse though since I was able to at least run the tests locally with -PflinkConfDir=$pwd/runners/flink/src/test/resources.

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[asf-ci]

Can one of the admins verify this patch?

[asf-ci]

Can one of the admins verify this patch?

[asf-ci]

Can one of the admins verify this patch?

[damccorm]

Run XVR_Flink PostCommit

[damccorm]

Run XVR_Flink PostCommit

[damccorm]

Run Go Flink ValidatesRunner

[damccorm]

Run Go Flink ValidatesRunner

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[lostluck]

Sounds good. Ping me if a revert is needed. Thanks!

[damccorm]

Run XVR_Flink PostCommit